Optical bleaches or brighteners are major chemical products. Many of these compounds are used in major amounts.
They are either added to the polymer component mixes before fiber and sheet formation or are coated unto the fibers, fabrics or pellicles formed therefrom. In addition fabric and textiles are also dyed with these products. Major amounts of these materials are also added to laundry washing compositions to impart brightening qualities to the laundry washed therein.
A major intermediate for the manufacture of many of these brighteners is 4,4'-diaminostilbene-2,2'-disulfonic acid. Optical bleaches and brighteners according to U.S. Pat. Nos. 2,762,801 and 3,472,842 are marketed by Ciba-Geigy Corp. under the trademark TINOPAL. Specifically, the optical brightener, disodium 4,4'-bis-[2"-phenylamino-4"-(N-methyl-.beta.-hydroxy-ethylamino)-s-triazin yl-(6")amino]-stilbene-2,2'-disulfonic acid is a thermally stable compound used in detergent, soap an similar laundry composition. These optical brightening washing compositions are used in commercial laundries and also in the home for the greater "whiteness" in white goods and also for enhancing the brilliance of the colored goods.
A major intermediate in this connection is the 4,4'-diaminostilbene-2,2'-disulfonic acid (DAS).
Diaminostilbene (DAS) has been prepared commercially by the Bechamp Reduction process (Merck Index, 10th Ed. pg. ONR-8) directed to the reduction of aromatic nitro compounds to the corresponding amines by iron or ferrous salts in aqueous acid.
The literature on this reaction indicatss acetic and hydrochloric acid to be commonly used. Until now DAS has been prepared from 4,4'-dinitrostilbene-2,2'-disulfonic acid by reduction with iron in aqueous acetic acid solution. While the literature indicates hydrochloric acid has also been used to activate the iron in Bechamp reductions it would not be useful for diaminostilbene production, as the corrosion rates are very high requiring the use of expensive corrosion resistant equipment such as glass lined reactors, filters and piping. Less expensive stainlss steel equipment is satisfactory for sulfuric acid service under the conditions used in the invention. Additionally, hydrochloric acid is more expensive than sulfuric acid.
In the reduction of 4,4'-dinitrostilbene-2,2'-disulfonic acid it is very important to maintain an acidic pH, most preferably in the range of 5-6 to avoid the formation of highly colored and undesirable sun yellow by-products. Acid is required in this invention for reaction control during the reduction as well as for activation of the powdered iron.
Tests to manufacture the diaminostilbene with sulfuric aacid have shown that sulfuric acid is less efficient than acetic acid in this process. The reaction rates are sufficiently lower to obviate any cost advantage lying in the use of the cheaper sulfuric acid. Using acetic acid, adjusted to the optimum pH of 5-6 by the addition of NaOH, the reduction is completed within about 30 minutes. With sulfuric acid alone substituted for the acetic acid, the reduction requires approximately 60 minutes under the same conditions. Attempts were made to approximate the pH and the salt containing conditions of the acetic-acid sodium-hydroxide process by the introduction of NaHSO.sub.4, Na.sub.2 SO.sub.4 and NaOH into the sulfuric acid. While slightly decreasing the reaction time to approximately 45 minutes, the production rate was still substantially slower than the acetic acid/sodium hydroxide process.